1. Field of the Invention
This invention relates to tubular articles and methods of making the same and more particularly to such articles suited for air, gas or liquid flow and/or storage. It also relates to the art of compression molding reinforced thermosets and more particularly to directing resin flow by controlled air displacement during the molding process.
2. Description of the Prior Art
It has heretofore been proposed in the Sipler U.S. Pat. Nos., 2,990,855 and 2,995,781 to provide tubular conduits and methods of making the same which utilize the mechanical interactions between an inflatable mandrel and a surrounding knitted fabric resin carrying component to produce monolithic permanently shaped strong non-porous tube-like objects by a low pressure molding operation self-stopped by setting of the resin, the fabric being retained in the body of the finished article to contribute to the performance of the finished conduit. Such conduits have been extensively used in the automotive field on trucks for connecting air cleaners to carburetors and for other purposes and are tailored by choice of resin, filler and continuous, surrounding reinforcement so that each unit volume of the tubular articles produced described by the aforesaid one-shot, self-stopped molding operation can contribute substantially equally to the performance of the whole. The premolding reinforcement assembly and immersion resination operation essentially distribute these materials with circumferential symmetry and lengthwise uniformity thus primarily require the otherwise unaided directional stretch and incompressibility of rib-knit reinforcement to determine resulting wall thickness, wall thickness uniformity and strength throughout the overall tortuous conformations of such articles and their local changes in shape.
Each of the three steps of the Sipler method is both essential for the operation of the method and critical to the characteristics and uses of its products as conduits. Trapped air weakens such products. Continuous bubble paths can cause leaks. In this particular method as in all related methods for producing fiber reinforced thermosets without vacuum assistance, resin is caused to flow by directed compression pressure. Liquid resins are essentially incompressible and can only flow by their displacement of other resin or air. The Sipler method has neither external nor internally localized means for controlling these key flows and its self-controlled patterns of compression pressure distribution are complex. Consequently, considerable resin losses are accepted in order to insure the satisfactory quality of its tortuously shaped products.
None of several other methods now known for producing fiber reinforced thermosets can produce tortuous conduits. Each is characterized by a relatively simple pattern of compression pressure distribution and provides for its essential air displacements in ways which are not only relatively simple but also can remain quite inconspicuous until they cause production problems and product deficiencies of the same kinds as have long plagued the fiber reinforcement of thermosetting resins by hand lay-up or any of its more sophisticated filament and/or tape winding analogs. In all of these latter methods, proper management of air displacement is well known to be critical.
In order to better recognize the peculiarities of this common problem in the Sipler method and perceive the ways the present invention handles them in this special case, it will be helpful to summarize the related teachings of others in the following ways.
Chant, in British Pat. No. 1,266,097, uses a complete, resin-carrying, open cell foam layer as the key, integrated device of a new method for producing sheet laminates. Functionally, the Chant method is a geometrical simplification of the Sipler method. Chant replaces the complete outer Sipler fabric tube with a complete foam layer. All resin is uniformly introduced and provided by that layer. Solid platen compression is applied uniformly. Thermosetting heat is uniformly contacted by resin flow. Resin flow patterns are simplified by air displacement from all open edges of the sandwich laminate. Product shapes are flat and simple. Sheet thickness is determined by mold stops. A subsequent modification of this method shown in U.S. Pat. No. 3,867,221, to produce lower density sheets requires a complete layer of high rebound foam, a controlled decompression step and the uniform, partial reexpansion of this resin containing foam layer to trap a layer of foam-reinforced void space.
Neither Chant method provides any localized means for controlling air displacement. In effect, Chant's low density modification recognizes trapped void problems encountered in the operation of its precursor.
The low density Chant method does demonstrate that the viscosity gradient of a solidifying thermosetting resin system from a hot surface through a layer of resin-impregnated reinforcement is sharp. In no way does it or can it use its complete foam component, dry, as either a local heat insulator an temporary air valve or, wet, as a means for immobilizing its load of resin sufficiently to create a local source of pressure directable fluid flow and/or flow stoppage by thermoset skin formation against a hot mold surface. In short, Chant's simplification of the Sipler method is not operable for the far more complex process of producing tortuous, tubular shapes in an unstopped hot mold by single inflation of an expansible internal mandrel. Reinforced shroud laminates produced by obvious hot press adaptations of the Sipler method of the production of flat shapes have been available on the commercial market for several years and are not the subject of the present invention.
The pervading problem of air displacement is effectively handled by the Wiltshire method in U.S. Pat. No. 3,177,105, for producing reinforced tanks by the expansion of a supported, inflatable bag against a rigid pre-loaded unsplit outer mold. In a key, preliminary and in situ step heavy liquid resin is slowly pumped up into the confined and preassembled reinforcement layer to force and displace all of the lighter air upwards where it is vented from the top of the molding apparatus. Then the bag is inflated to form this air-freed and incompressible liquid-solid layer into its tank shape.
Both the Sipler method and all of the preferred embodiments of present improvements of it accomplish both the air displacement step and the product shaping step by one single, very rapid inflation of a pressurizing mandrel against a resinated assembly laid into a horizontally split mold. At the moment of mandrel inflation, Sipler's split, confining outer mold contains a considerably larger volume of air than of incompressible liquid resin and solid reinforcement. Air is free to exit along lengthwise mold lands and at the ends of the mold. In this particular setting, no adaptation of the single-direction, forced-gravity preliminary air displacement operations taught by Wiltshire is possible.
In a negligible pressure, ultraviolet activated method for molding a pre-wound load of thermoplastic tape to form a leg cast, Asbelle et al., in U.S. Pat. No. 3,823,208, use specially placed felt patches to provide the localized comfort of felt-air cushions. Specifically, no resin flows into, out of or through these patches. Always positioned within the multiple layer sock and tape assembly, they are precemented in place, integrated only by being surrounded and in no way participate in any resin flow control. For the entirely different purposes of the present invention, supplemental localized foam, felt and/or fabric components always participate in fluid flows and finally are integrated by impregnating resin.
In the Sipler method, all reinforcing components are loaded onto an inner tube mandrel, the assembly is trough coated with catalyzed thermosetting resin and this resinated assembly is placed into a simple split and rigid hot metal mold for single inflation forming. The surrounding hot mold determines the outer shape of the product. The inner shape of the product and thus both its wall thickness and its cross section for conduit use are determined by the local expansion of the mandrel against the constraints of its uniformly continuous load of expansible reinforcement and rapidly reacting resin matrix. There are no mold stops in this method. Wall thickness is entirely determined by the stretching and flowing interactions of its integrated components with an expandable mandrel which distorts locally when inflated so as to shape itself to the fixed shape complexities of the surrounding, rigid mold.
In the few seconds required for the mandrel to attain its final molding pressure, two major and critical fluid flows take place within the stretching, resinated reinforcement assembly. Air is displaced from the mold and liquid resin follows the air it is pressed against the flow through and fully impregnate the reinforcement and form the conduit product. The air which surrounded the resinated assembly when it was placed into the mold is mainly forced out through the parting lines of the mold and the air not previously displaced by resin within the resinated reinforcement assembly mainly exhausts along the expanding mandrel and out the open ends of the mold. The now relatively continuous layer of incompressible liquid resin follows both air flows until it is either trapped in the pressure packed reinforcement or stopped by the rapid increasing of its own viscosity by the heat of the surrounding mold or is wasted from the mold parting lines and its open ends.
Although the external operations of the Sipler method are simple, its internal performance is complex and becomes increasingly so as the necessary shapes of its products become more tortuous lengthwise and involve sharp changes in cross section size and/or shape. Increasing the sufficient excesses of resin is not always the most practical solution. Localized internal control of its air and resin flows is possible and practical and is accomplished by the instant invention. With minimum other operating changes, the primary air displacement patterns of the Sipler method are modified so as to utilize resin more efficiently.
As in all thermoset molding and, in part, because set resin is neither recoverable nor reworkable, the major wastes of resin are in rejected products and in resin distribution in acceptable products which are less than optimum for meeting the performance requirements of the product. In the first instance, all of the material, labor and time are lost. In the second, the high setting shrinkage of thermoset resins makes the shrink crackage of thick resin-rich pools a special problem to the Sipler method because its inflated mandrel expands its uniformly surrounding load of fabric and resin with considerable local variation in rates and extents to obtain tortuous and complexly shaped conduits. In service, the inner surfaces of conduits are highly important. In production, these surfaces are the most difficult to inspect.
Some resin must be wasted through outer mold lands in order to insure sufficient and satisfactory amounts are everywhere retained in product walls. With the uniformities of sock loading and resination peculiar to the Sipler method, minor amounts of such land flashing are, at present, the best insurance of satisfactory overall completion of each conduit by the molding process. In large measure and in the absence of any external mold stops in this self-stopped method, it is the packing of stretching fabric which determines local wall thickness as the mandrel expands and shapes itself and its surrounding fabric-resin load to the contours of the outer mold. During the very brief period when resin is sufficiently liquid to flow and accomplish the full impregnation of all fabric components, it is the only incompressible fluid in this solid-liquid-air system. Thus it flows when the system is compressed and stretched and always flows to follow the air it displaces.
The primary purpose of the present invention is to more favorably control those air displacement flows, locally, so as to permit safe and satisfactory decreases in the total amount of resin required to make each particular conduit. In effect, the air closed into the Sipler mold is not only at the same time both the cheapest and most costly raw material acted on by the method but also the most manageable one. Properly chosen and pre-filled with air (dry) or resin (wet), relatively small pieces of compressible open porous foam, felt and/or fabric are localized in main air displacement routes so as locally to optimize following flows of liquid resin when the mandrel expands. Primarily, the improvement taught here improves upon the peculiarities of the Sipler method. More generally, it is useful to other methods of thermoset molding by expansible mandrels.